Process for forming an image on insulative materials

ABSTRACT

A PROCESS FOR FORMING AN IMAGE ON AN INSULATIVE MATERIAL COMPRISING THE STEPS OF: DISPERSING ELECTROCONDUCTIVE POWDER ON AT LEAST ONE SURFACE OF SAID INSULATIVE MATERIAL AND ELECTROSTATICALLY CHARGING THE SAME NO LATER THAN SAID DISPERSING TO THEREBY FORM A LAYER OF SAID ELECTROCONDUCTIVE POWDER ON SAID INSULATIVE MATERIAL; DISPERSING PHOTOCONDUCTIVE PARTICLES ONTO SAID LAYER OF ELECTROCONDUCTIVE POWDER AND ELECTROSTATICALLY CHARGING THE SAME NO LATER THAN SAID DISPERSING TO THEREBY FORM A LAYER OF SAID PHOTOCONDUCTIVE POWDER ON SAID LAYER OF ELECTROCONDUCTIVE POWDER; SUBJECTING THE THUS FORMED LAYER OF PHOTOCONDUCTIVE PARTICLES TO IMAGEWISE EXPOSURE OF THE ORIGINAL IMAGE; REMOVING SAID PHOTOCONDUCTIVE PARTICLES IN THE EXPOSED AREAS; AND FIXING THE REMAINING PHOTOCONDUCTIVE PARTICLES TO THEREBY OBTAIN AN IMAGE ON THE SURFACE OF SAID INSULATIVE MATERIAL.

y 23, 1974 YASUQ TAMAI 3,825,421

PROCESS FOR FORMING AN IMAGE ON INSULATIVE MATERIALS Filed Oct. 29, 19712 Sheets-Sheet 1 FIG. I

y 1974 YASUO TAMAI 3,325,421

PROCESS FOR FORMING AN IMAGE ON INSULATIVE MATERIALS Filed Oct. 29, 19712 Sheets-Sheet 2 United States Patent Office Patented July 23., 1914U.S. CI. 96-1 R 14 Claims ABSTRACT OF THE DISCLOSURE J A process forforming an image on an insulative material comprising the steps of:

dispersing electroconductive powder on at least one surface of saidinsulative material and electrostatically charging the same no laterthan said dispersing to thereby form a layer ofsaid electroconductivepowder on said insulative material;

dispersing photoconductive particles onto said layer ofelectroconductive powder and electrostatically charging the. same nolater than said dispersing to thereby form a layer of saidphotoconductive powder on said layer of electroconductive powder;

subjecting the thus formed layer of photoconductive particles toimagewise exposure of the original image;

removing said photoconductive particles in the exposed areas;'and

fixing the remaining photoconductive particles to thereby obtain animage on the surface of said insulative material'.

BACKGROUND OF THE INVENTION Field of the Invention This inventionrelates to an improvement in electrophotographic processes, andparticularly relates to an improvement in an electrophotographicscribing or image formation process on an insulative material utilizingphotoconductive particles as the photoconductive element therein.

Description of the Prior Art One variation of standardelectrophotographic processes recently proposed is a process utilizing acombination of a photoconductive powder containing a transparent coreand an electroconductive plate (for example the process disclosed inlap. Pat. Pub. 12,385/ 69). This electrophotographic process consists ofthe steps of first dispersing a photoconductive powder and thenelectrostatically charging the photoconductive powder, or elsedispersing a previously charged photoconductive powder, followed byadditional charging if desirable, on an electroconductive supportmaterial, subjecting the thus dispersed and charged powder to imagewiseexposure of an original image, removing the powder of which theelectrostatic charge is dissipated or decreased by exposure, andeither-fixing the remaining powder in situ or after transfer thereof.The photoconductive powder used consists of a photoconductive materialand powder core material provided therein having an absorptioncoefiicient not exceeding 1.3)( mm.- for the major part of the radiationin the sensitive wavelength region of the photoconductive material. Thisprocess is capable of reproducing clear, sharp images onelectroconductive plates and is employed in electrophotographicplatemarking apparatus for scribing onto steel plates etc.

The above-mentioned process, however, is incapable of reproducing imageson insulative materials.

'Thepresent inventors have provided a novel imaging process capable ofobtaining images on the surface of insulative materials.

SUMMARY OF THE INVENTION The present invention relates to anelectrophotographic scribing process for insulative materials comprisingthe steps of:

either dispersing electroconductive powder on an insulative materialsimultaneously with electrostatic charging thereon or dispersingelectroconductive powder on said insulative material which previouslyhas been charged, thereby forming thereon a layer of saidelectroconductive powder;

dispersing a photoconductive powder on said layer followed byelectrostatic charging thereon, or dispersing said photoconductivepowder which has been previously charged onto said layer, followedeventually by additional charging if desired;

subjecting said dispersed photoconductive powder to imagewise exposureof an original image;

removing said photoconductive powder in the areas where theelectrostatic charge thereon is dissipated or decreased by saidexposure; and

fixing the remaining photoconductive powder.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view of anexample of photoconductive particles employed in the process of thepresent invention;

FIGS. 2-8 are schematic side-views showing the steps of a scribingprocess for an insulative material according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION The insulative material employedin the invention can be in any shape, e.g., a fiat plate. It will beapparent to one skilled in the art that the exact shape of theinsulative material does not form a part of the present invention.

The electroconductive powder to be dispersed on said insulative materialcan be a metal powder, metal spheres, a resin powder covered with metal,electroconductive carbon spheres, a resin powder covered withelectroconductive carbon, glass spheres covered with anelectroconductive tin oxide layer, cuprous iodide etc. In short, anypowder, be it homogeneous or composite, which is capable of carrying anelectric current without degradation may be used in the presentinvention. The electroconductive powder preferably has a diameterranging from 5 microns to 2 millimeters.

The preferred electroconductive powders have a conductance greater than1x10 v. cm. specific surface conductance (the conductance determinedwhen electrodes having 1 unit length are disposed in parallel havingwith a distance 1 unit between them) when the powders are laid on theinsulating base. The most preferred specific surface conductance rangeis ca. 10 -10 v. cm. but it is to be understood such range is. notmandatory.

In addition to those particles set out above, particles having adiameter of 50 to 500p. such as'glass balls, siliceous sand, alumina,china and the like coated on their surface with a mixture of carbonblack and a resin, or

3 der on the insulative material which has been previously charged, orby dispersing the electroconductive powder simultaneously withelectrostatic charging.

It is, of course, necessary to disperse the powders to have good contactwith each other. This depends on the particle size, however, it is,generally preferred to have a thickness, of three or four layers ofclosely packed particles. Thicker layers, of course, can be used. Theminimum thickness generally used is about 20p, as this insures anacceptable necessary specific surface conductance to provide goodresults.

The process according to this invention will now be further explainedwith reference to the attached drawings wherein:

FIG. 1 shows a cross section of one example of a photoconductive powderparticle employed in this invention.

The photoconductive particle 35 is composed of a transparent core 11 anda photoconductive insulative layer 12 thereabout. The transparent core11 preferably has an absorption coefficient not exceeding 1.3 mm.- forthe major part of the radiation in the sensitive wavelength region ofthe photoconductive layer 12 (for best results a $50 m shift from thepeak). To facilitate the fixing step it is most preferred that at leastone of the core material 11 or the insulative photoconductive layer 12contains a thermoplastic resin material or a material which is solublein a suitable solvent which can be applied to achieve fixing.

Suitable solvents for fixing will, of course, depend on the material tobe marked and the resinous material, if used, in the photoconductivepowder; generally speaking, trichloroethylene, tetrachloroethylene,acetone, MEK, MIBK, ethyl acetate, propyl or butyl acetate, toluene,xylene are used.

Representative of preferred photoconductive particles for use in thepresent invention are those disclosed in British Pat. No. 1,165,017.

Usually the photoconductive particles will have a size of 10 to 200microns, with a size of to 80 microns being most preferred.

The thickness of the photoconductive particle layer is generally inaccordance with such layers as are known to the art, and must besufficient to permit the layer to perform its image-yielding function.Usually a thickness of at least two of the smaller sized photoconductiveparticles is required, and thus the preferred photoconductive layershave a thickness of 20 to 200 microns, though obviously thicker layerscan be used. For instance, using particles of a specific gravity of 1.3as disclosed in the above British patent a layer thickness of over 20microns is used.

One skilled in the art will appreciate that it is necessary to chargesuch a photoconductive layer to a degree sufficient to enable imageformation, and will be able to easily determine such charging values. Asurface voltage of about 350 volts or greater is generally used.

Preferred thermoplastic resin materials include polymethylmethacrylate,polystyrene, poly a-methylstyrenc, polyamide, polycarbonate,polyvinylchloride, polyvinylbutyral and the like, which are polymerizedor condensed polymers having a secondary transition point higher thanroom temperature, e.g., in the range 80 to 200 C.

FIG. 2 shows the step of electrostatic charging on the surface of aninsulative flat plate 14 on which the image is to be reproduced. Theinsulative plate 14 is placed on an electroconductive support plate 13and exposed to corona discharge generated by corona electrode 15provided in shield case 16 moving above the surface of plate 14. Thecorona electrode 15 is externally supplied with a high electricpotential (a negative potential in this case). Electrostatic chargingcan also be carried out by other charging processes known to the artsuch as frictional charging. The exact charging method'is not of greatimportance.

Electroconductive powder 22 is dispersed as shown in FIG. 3 onto thesurface of .thus charged insulative plate 14 from a reservoir 21 to forma uniform layer.

FIG. 4 shows the dispersion of electroconductive powder 22 on thesurface of an insulative material of irregular shape. In this case theelectroconductive powder 22 contained in a container provided with anappropriate outlet thereunder is made .to fallonto the surface of thematerial 31 in a layer and is simultaneously exposed to a coronadischarge generated by the corona electrode 15, thereby being depositedin a uniform layer on the surface thus charged. v 7

FIG. 5 shows the step of dispersing photoconductive particles 35 on theinsulative material 14 which already is provided with a layer ofelectroconductive powder 22 by means of the step explained in FIG. 4.The photoconductive particles 35 supplied from the conta ner areelectrostatically charged by means of corona discharge electrode 33 anddispersed on the layer of electroconductive powder 22. Uniformdispersion of said photoconductive particles can be accomplished, forexample, by the dispersing device disclosed in lap. Pat.' Pub. 8,838/70in which the powder is dispersed as solid-gas sol. It is naturally alsopossible to apply electrostatic charging to said particles afterdispersion thereof.

FIG. 6 shows the step of development with air et after imagewiseexposure thereon.

The photoconductive particles 35 in the exposed area B lose theirelectrostatic charge and are eliminated by the air jet supplied by duct36 displaced in the direction of arrow. 35 indicates the photoconductiveparticles thus removed by the air jet and collected in a duct 37 ofreduced pressure.

In this case the electroconductive powder 22 electrostatically depositedon the insulative material 14 1s not removed therefrom by the air jet,and the photoconductive particles 35 in the unexposed area A remain onthe layer of electroconductive powder as these particles still retainelectrostatic charge. The air jet supplied from the duct 36 is regulatedso as to not remove the photoconductive particles in the unexposed area.I. g

It is important that the electroconductive powder 22 be firmly retainedon the insulative material 14, since the separation of this powder fromthe recovered photoconductive particles 35' at the cycled use thereof.Forthis purpose the insulative material 14 is required to be highlyinsulative, and preferably has a resistivity of about 10 ohm.cm orhigher most preferably 10 ohm.cm or higher. At lower resistivity, theconductive-powder cannot be retained stably on the surface of insulativema: terial 14 and is apt to contaminate therecovered photoconductiveparticles. To allow forthe case where such might happen, it is preferredto have a difference in specific gravity or magnetic properties betweenthe electroconductive powder and the photoconductive particles tofacilitate the separation thereof. 3 r w u w FIG. 7 shows the step offixing the image consisting of the photoconductive particles by means ofa fixing head. displaced in the direction of the arrow after theabovementioned steps. A solvent 42 contained in the container 41 issprayed in mist form 44 from a nozzle 43. The component soluble in thesolvent (the resinous component in most cases) in the photoconductiveparticles is dissolved and firmly sticks onto the surface of insulativematerial 14 together with the photoconductive particles in the unexposedarea A to form the fixed image 45. If the electroconductive powder 22 iscomposed of a material insolu-l ble in the solvent, such as a me'tal,the electroconductive powder is not dissolved by the sprayed solvent andtherefore is not fixed onto the insulativematerial 14. whenI saidsolvent is evaporated. Thus, if the electroconductive powder 22 isdesired not to be fixed .onto the surface of the insulative material 14in thenon-image area B thereof, the solvent 42 is preferably one whichdoes not dissolve the insulative material 14 or der 22.

The electroconductive powder 22 sticking to the nonimage area B can beremoved by exposing the assembly to a corona discharge of invertedpolarity with respect to that employed in the step shown in-FIG. 2 or ofan alternating polarity, to thereby eliminate the electrostatic chargeremaining thereon. An'air jet may then be used to remove'the powder 22at area B. Mechanical elimination of the charge and removal by means,for example, of a brush are among alternative removal procedures.

Charge elimination can also be achieved by the presence of a smallamount of non-film-forming antistatic agent in the solvent 42 employedin the step of FIG. 7, or by passing the insulative material throughgrounded electroconductive particles as explained in lap. Pat. Pub.3,669/ 69.

FIG. 8 shows the image 45 containing the electroconductive powder 22thus formed on the insulative material 14 according to the process ofthis invention thus far explained. Thus this invention enables one toelectrophotographically obtain an image on the surface of an insulativematerial.

The process of this invention is applicable to insulative materials andmaterials provided with an insulative surface, such as a metal plateprovided thereon with a resinous layer, a metal plate provided thereonwith an .enameled layer, moulded plastics, plastic sheets, glass plates,glass blocks, glass bottles, etc.

The above material represents a sufficient disclosure to completelyenable one skilled in the art to practice the invention. The followingExamples are merely ofiered to show two typical processings inaccordance with the present invention.

the electroconductive pow- Example I On an electrostatically chargedpolyethylene terephthalate film, a finely divided black powdercomprising parts of carbon black and 10 parts of phenol resin, having amean particle diameter of about 5014 was cascaded. The charging of thefilm was carried out by a negative corona. The black conductive powderuniformly adhered on the film surface. The weight of the powder layerwas 25 g./m. 30 g./m. and the layer showed a surface resistance of about10* cm./square. On this black layer there was again spread aphotoconductive powder which had been prepared according to the methoddescribed in Example I in British Patent 1,165,017. The coated weight ofthis second layer was about 85-110 g./m. The covered film was subjectedto a negative corona with its carbon layer connected to ground. Theinitial potential of the top was 400 volts. An optically positive imagewas projected and the film was subjected to an air stream. Whereby thephotoconductive powder remained only at the non-irradiated area giving awhite image against a dark background.

Fixing of the image was carried out by spraying cyclohexane onto thefilm whereby the conductive powder did not dissolve at all. Then afterthe evaporation of the solvent, the conductive powder at the backgroundarea was eliminated.

Example II A piece of glass plate was covered with silver coated smallglass beads having a diameter of about 50m with silver chemically platedthereon. The coated weight of the beads was 40 g./m. On this coatingthere was applied the photoconductive powder described in Example VIIIin the above cited British patent. The coating weight was about 150g./m. When this sheet was exposed to a negative corona an initialsurface potential of about 500 volts was obtained. Then an air streamwas carefully applied onto the plate to glow off only thephotoconductive powder at the irradiated portion. The photoconductivepowder image was then fixed by spraying trichloroethylene thereon. Afterthe evaporation of the solvent, the conductive powderwas removed bybrushing.

What is claimed is: i

1. A process for forming an image on material comprising the steps of:

dispersing electroconductive powder on at least one surface of saidinsulative material and applying electro static charge to the powder nolater than said dispersing to thereby firmly retain but not physically:bind said electroconductive powder on'said insulative material; Idispersing photoconductive particles onto said layer ofelectroconductive powder and electrostatically charging thephotoconductive particles to thereby form a layer of saidphotoconductive powder on said layer of electroconductive powder;subjecting the thus formed layer of photoconductive particles toimagewise exposure of the original image; removing said photoconductiveparticles in the exposed areas; and fixing the remaining photoconductiveparticles to thereby obtain an image on the surface of said insulativematerial.

2. The process of claim 1 wherein said insulative material has aresistivity of at least about 10 ohm.cm.

3. The process of claim 1 wherein said electroconductive powderdispersed on said insulative material layer has a specific surfaceconductance of greater than about l l0 v. cm.

4. The process of claim 1 wherein said photoconductive particlescomprise a photoconductive insulative layer about a transparent corehaving an absorption coefiicient no greater than 1.3)(10 mm.- for themajor portion of the sensitive wave length region of the insulativelayer.

5. The process of claim 4 wherein at least one of said core or saidinsulative layer contains a thermoplastic resin.

6. The process of claim 1 where the first charging is prior todispersing the electroconductive powder.

7. The process of claim 1 where the first charging is simultaneouslywith dispersing the electroconductive powder.

8. The process of claim 1 Where the second charging is subsequent todispersing the photoconductive powder.

9. The process of claim 1 where the second charging is prior todispersing the photoconductive powder.

10. The process of claim 9 where the second charging is prior todispersing the photoconductive powder, and after said dispersing saidelectroconductive powder is additionally charged.

11. The process of claim 1 where the second charging is simultaneouslywith dispersing the photoconductive powder.

12. The process of claim 1 wherein said electroconductive particles aredispersed in a layer at least about 20 microns thick, and saidphotoconductive particles are dispersed in a layer at least about 20microns thick.

13. The process of claim 1 wherein said photoconductive particles arecharged to a surface voltage of at least about 350 volts.

14. A process for forming an image on an insulative material having aresistivity of at least 10 ohm.cm.,comprising the steps of:

forming a conducting layer of an electroconductive powder on one surfaceof said insulative material, said conducting layer having a specificsurface conductance greater than 1x10 v. cm. and being formed ofparticles of a size of from 5 microns to 2 millimeters at least threelayers deep;

at a point no later than the forming of the conducting layer, applyingelectrostatic charge to the conducting layer to firmly retain but notphysically bind the layer on said insulative material;

an insulative forming a layer of photoconductiveparticles of athickriess of 10 to 200 microns on the thin'formed and chargedconducting layer, said layer of photoconductive particles having athickness of atleast 20 microns; I g

electrostatically charging said layer of photoconductive particles to asurface voltage of greater than 350 volts at apoint no later thansubjecting the layer of photoconductiveparticles to imagewise exposure;

imagewise exposing the thin charged layer of photoconductive. particlesto an original image;

removing the photoconductive particles in the exposed areas; and

fixing the remaining photoconductive particles to there by obtain animage on the surface of said insulative material.

v References Cited UNITED STATES PATENTS 2,922,519 2/1960 Bertelsen 96-1R g g Y FOREIGN PATENTS 1,165,017 9/1969 'Cireat Britain 96 1R 1,082 9129/1967 Great Britain, 9 ,-1 R 990,538 4/1965 Great Britain '961 R38/22,645 1963 Japan 96-1 R 234,016 8/1959 Australia 96--1 R 1,008,63311/1965 Great Britain 961.5

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.

